Modern commercial aircraft have horizontal stabilizers that pivot relative to the airplane fuselage to "trim" the aircraft during flight. This involves adjusting the position of the horizontal stabilizer, to accommodate for different load distributions within the aircraft, for example. One common horizontal stabilizer trim actuator consists of a ball nut mounted in a gimbal at the leading edge of the center of the horizontal stabilizer structure, and an upright ball screw extending through the ball nut. The ball screw, in turn, has its end remote from the ball nut mounted in a gimbal secured to the fuselage. By rotating the ball screw in one direction, the leading edge of the horizontal stabilizer is moved up, whereas by rotating the ball screw in the other direction, the leading edge of the horizontal stabilizer is moved down. Rotation of the ball screw can be by a motor and associated gearing.
The horizontal stabilizer hinge moment is transmitted through the stabilizer gimbal and ball nut to the ball screw. This load has a vertical component as well as a torque component due to ball screw lead (threads). Toward the base of the ball screw at its junction with the fuselage, a "primary" no-back device is provided to apply a braking force during trimming. Preferably, the primary no-back device provides a force resisting rotation of the ball screw in a direction that would result in movement of the stabilizer in the direction of the applied aerodynamic force (called the "aiding direction"), while applying little or no force resisting rotation of the ball screw in the direction that would result in movement of the horizontal stabilizer contrary to the direction of the applied aerodynamic force (called the "opposing direction").
In known designs, the primary no-back device is located remotely from the motor and gearing which turn the ball screw to trim the horizontal stabilizer. In the case of failure of the primary no-back device, the ball screw would be backdriven due to the aerodynamic load on the horizontal stabilizer, and an extremely dangerous condition would result. Thus, it is necessary to have a secondary brake which prevents backdriving of the ball screw in the case of failure of the primary no-back device, but which provides limited resistance to driving of the ball screw to trim the horizontal stabilizer, regardless of whether the ball screw is being driven against the force of the aerodynamic load on the horizontal stabilizer (opposing direction) or in the same direction as the force resulting from the aerodynamic load (aiding direction).
An example of a bi-directional rotary brake is shown in Allan et al. U.S. Pat. No. 4,850,458. The device shown in that patent provides a braking force preventing either a reverse load or an overrunning load from turning an output shaft. A predetermined amount of lost motion is permitted between the input shaft and an input cam plate, and an output shaft and an output cam plate. Turning of the output shaft beyond the predetermined amount of lost motion causes relative rotation of the input and output cam plates. Steel balls mounted between the input and output cam plates ride in cupped sockets or "ramps" such that the relative rotation of the input and output cam plates forces them apart. This axial motion results in a compressive force being applied at the output side of the output cam plate and at the input side of the input cam plate. In each instance the compressive force increases frictional brake forces between the cam plate and the brake housing by way of a skewed roller brake disc. Thus, if the output shaft is driven in either direction relative to the input shaft, a braking action is achieved. While having some mechanical components similar to the present invention, the device disclosed in Allan et al. is not adapted for use as a secondary brake in a horizontal stabilizer trim actuator, particularly in view of potential brake chatter and difficulty in adjusting brake gain.
Other secondary brakes in horizontal stabilizer trim actuators have used ratchet-pawl constant drag brakes which require more frequent service than desirable, somewhat inconsistent brake torque during the useful life, and potential mechanical failure, such as skipping teeth between the ratchet-pawl couplings.